The present invention relates to an improved construction of a vertical full rotary hook for a sewing machine where an inner rotary hook is assembled into an outer hook for relative rotation thereto.
FIGS. 10 and 11 show a typical example of the conventionally-known vertical full rotary hooks for sewing machines. The rotary hook shown in FIGS. 10 and 11 comprises an outer rotary hook 21 fixed to a lower shaft 20 rotatable in synchronism with ascending and descending (i.e., up-and-down) movement of a sewing needle, and an inner rotary hook 22 fitted in the outer rotary hook 21. The outer rotary hook 21 has a blade point 23, and the outer rotary hook 21 is driven to rotate in one direction in response to rotation of the lower shaft 20. The inner rotary hook 22 is fitted in the outer rotary hook 21 in such a manner that it is rotatable relative to the outer rotary hook 21 within the outer rotary hook 21. The inner rotary hook 22 has an engaging portion 24 on an outer peripheral region of the front surface thereof, and the engaging portion 24 engages with a rotary hook supporting projection (not shown), fixedly provided on a machine frame, so as to prevent the inner rotary hook 22 from rotating with the outer rotary hook 21. The inner rotary hook 22 has an elongated needle drop hole 25 formed therein. A bobbin case 26 with a bobbin having a lower thread wound thereon is settable within the inner rotary hook 22.
In such a conventionally-known vertical full rotary hook rotary hook, as shown in FIG. 12, a loop L of an upper thread portion, formed rearward of the sewing needle 27 immediately after the sewing needle 27 has started ascending from its bottom dead point, is captured by the blade point 23 of the rotating outer rotary hook 21. Then, after the upper thread has gone around to (i.e., circled around) behind the inner rotary hook 22 in response to the rotation of the outer rotary hook 21, it is removed from the rotary hook by being lifted up via a well-known thread take-up lever at predetermined timing. In this manner, the upper and lower threads intertwine with each other to form a stitch.
When the upper thread is to be captured by the blade point 23 of the outer rotary hook 21 at the time of formation of a stitch, it is necessary that an appropriate upper thread loop L be formed as shown in FIG. 12. However, upper thread portions located forward and rearward, respectively, of the sewing thread 27 may both be undesirably looped as shown in FIG. 3 depending on properties of a fabric and thread used, and thus, the upper thread may be undesirably cut or broken by both of the upper thread portions being captured by the blade point 23.
A more sophisticated vertical full rotary hook designed to avoid such an inconvenience is disclosed in Japanese Utility Model Application Laid-open Publication No. HEI-5-39483 (hereinafter referred to as “the patent literature”), in which a sewing needle insertion groove of a U shape as viewed in plan is provided over the needle drop hole and in which the front and left and right sides of the sewing needle having descended are surrounded by a wall surface of the sewing needle insertion groove. In the disclosed rotary hook having the sewing needle insertion groove, where the three sides, i.e. front and left and right sides, of the sewing needle having descended are surrounded by the wall surface of the sewing needle insertion groove, the upper thread portion located forward of the sewing needle can be prevented from circling around behind the sewing thread 27. In this way, it is possible to prevent both of the upper thread portions from being captured by the blade point 23 of the outer rotary hook and thereby prevent breakage of the thread.
Further, in the vertical full rotary hook disclosed in the patent literature, which has the sewing needle insertion groove over the needle drop hole, the opening end of the sewing needle insertion groove is located substantially in alignment with the center line of the sewing needle as shown in FIG. 2 of the patent literature. However, the opposed side wall surfaces of the insertion groove can only prevent the upper thread portion, located forward of the sewing needle, from circling around behind the sewing needle, and thus, the opposed side wall surfaces of the insertion groove does not at all control the other or rear-side upper thread portion, so that the upper thread passed through the sewing needle may sometimes undesirably deviate or stray laterally depending on the twist of strands of the thread. Consequently, as the sewing needle starts ascending from its bottom dead point under such a condition, the upper thread portion located rearward of the sewing needle 27 may form a loop L1 protruding laterally as shown in FIG. 14. As a consequence, there tends to arise the inconvenience that the loop L1 is not captured by the blade point 23 and fails to form a stitch, or the blade point 23 slightly touches the loop L1 and split the twisted strands of the upper thread to catch and break fibers of the upper thread.